Abstract

There is a wide range of MHD instabilities which can develop in tokamak plasmas. These instabilities can have ideal or resistive (tearing) character, can be driven by gradients of pressure, current or energetic particles distributions (in position and momentum space), and can reside inside the plasma or affect the plasma boundary significantly. Investigation of their characteristics is the first step towards their identification. The poloidal and toroidal mode number, the mode frequency, the mode amplitude and spatial structure are important features to describe an instability. Although such a plain description does not directly make any statement on stability, driving forces and growth rate or saturation of modes, it is basic information for such analysis and for validation of stability codes. In the following we describe experimental possibilities to determine the above mentioned mode characteristics, mode coupling and how to distinguish between ideal and tearing modes.